In charged-particle-beam (CPB) projection-exposure microlithography, a charged particle beam (e.g., an electron beam) is used to transfer a pattern, defined by a reticle, onto the surface of the sensitive substrate such as a semiconductor wafer.
Conventional electron-beam projection-exposure exhibits high resolution but is flawed by low throughput (i.e., number of wafers that can be projection-exposed per unit time). Various approaches have been investigated to improve throughput.
One current approach, commonly termed "hybrid" pattern-area exposure (also termed "cell" projection, "character" projection, or "block" exposure), utilizes a reticle that defines multiple repetitive portions of the overall pattern to be projection-exposed onto the substrate. Each portion when projected onto the wafer typically measures approximately 5 .mu.m.times.5 .mu.m. Transfer of the mask pattern occurs by repeatedly exposing a single small pattern on the reticle onto multiple locations on the wafer, using an electron beam.
Unfortunately, hybrid pattern-area exposure requires much time to accomplish, especially with respect to non-repetitive portions of the pattern. Consequently, compared to other methods, throughput with hybrid pattern-area exposure can be lower by about one order of magnitude when applied to the actual production of basic semiconductor integrated circuit devices (e.g., DRAMs, etc.). In addition, the microprocessors that have been the subject of the most recent intensive R&D and production efforts have little to no pattern repetition; hence, hybrid pattern-area exposure is of little utility for such applications.
Another conventional electron-beam approach is "reduction" (demagnifying) projection-transfer as disclosed, e.g., in Japanese Kokai patent document no. HEI 5-160012. This technique offers prospects of substantially higher throughput than hybrid pattern-area exposure, and can be employed in the manufacture of microprocessors and the like. Successive portions of the pattern, defined by the reticle, of an entire die or "chip" are sequentially irradiated with the electron beam. The resulting image of the irradiated portion of the reticle is reduced and transferred to the wafer by a two-stage projection lens through which the electron beam passes.
In reduction projection-transfer, an entire die cannot practicably be irradiated by the electron beam all at once (so as to transfer the entire die pattern at once). Hence, the die pattern is usually divided into multiple fields and subfields. The die is transferred by sequentially transferring the pattern portion defined by each subfield, during which any of various parameters of the electron-beam optical system can be changed as required for the particular subfield. The projected images of the subfields (each measuring, e.g., 250 .mu.m.times.250 .mu.m) are arrayed and "stitched together" on the wafer surface.
An example of a reticle as used for reduction projection-transfer is disclosed in U.S. Pat. No. 5,260,151. The reticle is referred to therein as a "grillage". The reticle is divided into multiple subfields each measuring approximately 1 mm.times.1 mm square. The subfields are arrayed in a checkerboard grid pattern on a thin Si membrane (500 to 2000 nm thick). The subfields are separated from one another by non-patterned boundary regions (each approximately 0.1 mm wide) called "skirts". The reticle also comprises orthogonally intersecting reinforcing struts (each approximately 0.1 mm thick) arranged so as to surround each subfield at the skirts. The pattern portion in each subfield is defined by a respective membrane configured such that an electron beam incident perpendicularly (orthogonally) to the membrane at an acceleration voltage of approximately 100 kV is scattered to some extent while most of the electrons in the beam are transmitted through the membrane. The struts reinforce the membranes and enable a reticle measuring, e.g., 200 mm.times.100 mm overall to be self-supporting.
During manufacture of a grillage reticle, a layer destined to become the membrane is formed on the surface of a relatively thick reticle substrate. Etching is used to remove, down to the membrane layer, most of the reticle substrate while leaving portions of the reticle substrate that form the struts. During such etching, large stresses are frequently concentrated in the membrane layer near regions where the struts intersect each other (i.e., at the corners of the subfields). Formation of certain pattern features (e.g., holes, etc.) at such corners can cause excessive release of stress which causes deformation or displacement of the features, and consequent deterioration of pattern accuracy or actual damage to the membrane.